Plastic sheets or films, such as polyvinyl chloride or polyethylene film, may not be torn directly along a straight or other regular line. Rather, when tearing forces are applied in opposite directions, such materials or films deform and stretch until the elastic limit is surpassed, whereupon a tear or separation starts. This tear originates anywhere in the area of the deformation, usually at the weakest point produced by the reduction in thickness, and the tear will not necessarily run normal to the tearing forces which are applied. Simply, it is virtually impossible to tear a plastic film in a straight and smooth line due to its plasticity.
The tearability of plastic sheet materials has been improved in a number of ways. These methods generally provide for an easy-tear arrangement in the film by the formation of score lines, either molded or mechanically formed in the film, alone or in combination with other tearing means. These easy-tear arrangements eliminate the need for a separate cutting tool and enable a person to tear the film along predetermined parting lines by hand. Furthermore, such easy-tear arrangements have been proposed in a number of products including bags, envelopes, adhesive tapes, packaging sheet materials or films, sizable products for shelves, walls, windows and other coverings.
Most generally tear lines are formed in plastic film by scoring or mechanically forming a groove in the plastic film. This groove may be formed by knife, chemical, solvent, extrusion, or other ways and devices. All of these techniques are directed to weakening the film along a predetermined path or line so that the plastic film is easy to tear. Such techniques of weakening necessarily reduce the overall strength of the sheet material itself. Such reduction in strength poses a number of problems or disadvantages in the end uses for the film. Other problems or disadvantages are common to a number of end uses for the scored sheet materials. For instance, in the case of known scored decorative wall coverings or window coverings for hand sizing on either a wall or in a window opening, a number of disadvantages exist. The score lines tend to be visible in the plastic sheet materials even though the lines are provided by very fine knives which cleanly penetrate the surface of the sheet material. Such visible score lines in a decorative product obviously detract from its utility and even diminish its salability. In addition, in the handling of such films, there is a problem of film fracture or splitting in the formation of the product, usually from larger supply rolls. Such splitting obviously renders useless the material from the supply roll and such material must be reprocessed at additional expense or scrapped. In use, when scored sheet materials are subjected to the environment, i.e., heat, sunlight or aging, these effects obviously are more pronounced on the thinned areas of the sheet material. Thus the lines of weakness intentionally provided for tearing the plastic also limit the useful life of the end product. The scores also provide areas for the accumulation of dirt or grime. End products are always subject to hazard of accidental tearing whereby they no longer may serve their useful function, as in the case of a window shade or decorative covering. In other areas of utilization such as the formation of bags, envelopes, adhesive tapes, packaging sheet material or films, sizable products for shelves, similar problems are encountered and, therefore, the utility of scored plastic films has been limited in these areas. There have been many efforts in the patent art to overcome these difficulties. Merely illustrative of such efforts are U.S. Pat. Nos. 2,705,579; 2,791,324; 2,811,280; 2,849,109; 3,186,628; 3,244,335; 3,379,814; 3,527,859; 3,563,839 and 3,616,990 and Great Britain Pat. Nos. 1,028,557; 1,088,564 and 1,294,652.